It is now common practice in the art to prepare libraries of genetic packages that display, express or comprise a member of a diverse family of peptides, polypeptides or proteins and collectively display, express or comprise at least a portion of the diversity of the family. In many common libraries, the peptides, polypeptides or proteins are related to antibodies. Often, they are Fabs or single chain antibodies.
In general, the DNAs that encode members of the families to be displayed and/or expressed must be amplified before they are cloned and used to display and/or express the desired member. Such amplification typically makes use of forward and backward primers.
Such primers can be complementary to sequences native to the DNA to be amplified or complementary to oligonucleotides attached at the 5′ or 3′ ends of that DNA. Primers that are complementary to sequences native to the DNA to be amplified are disadvantaged in that they bias the members of the families to be displayed. Only those members that contain a sequence in the native DNA that is substantially complementary to the primer will be amplified. Those that do not will be absent from the family. For those members that are amplified, any diversity within the primer region will be suppressed.
For example, in European patent 368,684 B1, the primer that is used is at the 5′ end of the VH region of an antibody gene. It anneals to a sequence region in the native DNA that is said to be “sufficiently well conserved” within a single species. Such primer will bias the members amplified to those having this “conserved” region. Any diversity within this region is extinguished.
It is generally accepted that human antibody genes arise through a process that involves a combinatorial selection of V and J or V, D, and J followed by somatic mutations. Although most diversity occurs in the Complementary Determining Regions (CDRs), diversity also occurs in the more conserved Framework Regions (FRs) and at least some of this diversity confers or enhances specific binding to antigens (Ag). As a consequence, libraries should contain as much of the CDR and FR diversity as possible.
To clone the amplified DNAs of the peptides, polypeptides or proteins that they encode for display on a genetic package and/or for expression, the DNAs must be cleaved to produce appropriate ends for ligation to a vector. Such cleavage is generally effected using restriction endonuclease recognition sites carried on the primers. When the primers are at the 5′ end of DNA produced from reverse transcription of RNA, such restriction leaves deleterious 5′ untranslated regions in the amplified DNA. These regions interfere with expression of the cloned genes and thus the display of the peptides, polypeptides and proteins coded for by them.